Spectra Of Crystals Research Articles

Bayesian Orbital Decomposition and Determination of End Condition for Magnetic Compton Scattering

In this paper, we have demonstrated the orbital component decomposition for the magnetic Compton scattering spectrum of Fe poly crystal by using Bayesian inference and the non-relativistic Hartree–...

The effect of Cr impurity and Zn vacancy on electronic and magnetic properties of ZnSe crystal

The spin-polarized electronic energy spectra of the ZnCrSe crystal were obtained based on calculations for a supercell containing 64 atoms. First, calculation is performed with an impurity of Cr atom, replacing the Zn atom. In the second variant, the Cr impurity and the vacancy at the Zn atom site are considered simultaneously. The results obtained in the first variant are as follows. It was found that the presence of the Cr atom leads to significant changes in the electronic energy bands, showing a large difference for different spin moments. The density curves of electronic states with opposite spins show an asymmetry, the consequence of which is the existence of a nonzero magnetic moment of the supercell. It was found that in the ZnCrSe crystal electronic 3d states with spin up are present at the Fermi level, i.e. the material is a metal. For spin-down states, the material is a semiconductor in which the Fermi level is inside the band gap. The value of the direct interband gap for electronic states with spin up is equal to 1.56 eV, and the magnetic moment of the supercell is 4.00 . The results obtained by the second variant of the calculation show a significant effect of the vacancy on the zinc site on the electronic structure of the ZnCrSe crystal. The Fermi level now intersects the dispersion curves of the upper part of the valence band for both spin orientations. The magnetic moment of the supercell is 2.74 .

Exciton dispersion in para-quaterphenyl: Significant molecular interactions beyond Coulomb coupling

We have experimentally determined the momentum dependence of the electronic excitation spectra of para-quaterphenyl single crystals. The parallel arrangement of para-quaterphenyl molecules results in a strong Coulomb coupling of the molecular excitons. Such crystals have been considered to be a very good realization of the Frenkel exciton model, including the formation of H-type aggregates. Our data reveal an unexpected exciton dispersion of the upper Davydov component, which cannot be rationalized in terms of inter-molecular Coulomb coupling of the excitons. A significant reduction of the nearest neighbor coupling due to additional charge-transfer processes is able to provide an explanation of the data. Furthermore, the spectral onset of the excitation spectrum, which represents a heavy exciton resulting from exciton–phonon coupling, also shows a clear dispersion, which had been unknown so far. Finally, an optically forbidden excitation about 1 eV above the excitation onset is observed.

P-108. Pregnancy-associated stroke – frequency and course of subsequent pregnancies

In this paper, the terahertz (THz) absorption spectra of a nematic liquid crystal (NLC) cell are estimated via THz time-domain spectroscopy (TDS) using a mode-locked Yb-doped fiber laser as the pump source. The THz-TDS system is implemented by using a dipole antenna as an emitter and a bow tie antenna as a receiver. A THz emission bandwidth exceeding 2.2 THz is achieved corresponding to a free spacing of 220 mm and a signal-to-noise ratio exceeding 30 dB. Subsequently, this system was utilized to measure and analyze the THz absorption spectrum of an E7 NLC cell by applying an electric field. The conclusions experimentally confirmed that the absorption of the specific THz frequency of the cell is inversely proportional to the intensity of the electric field applied to the NLC cell.

Abnormal negative thermal quenching of photoluminescence in CdMnTe:In crystals

Temperature-dependent photoluminescence (PL) properties of indium-doped CdMnTe (CMT:In) crystals were investigated. Four emission peaks of the tip crystal and three emission peaks of the tail crystal from as-grown CMT:In ingot were observed at low-temperature (10 K) PL spectra, respectively. From 20 K to 40 K, PL intensity of the deep defect-related emission peak ( D complex ) increased with temperature, showing a negative thermal quenching effect for the tip crystal. Interestingly, from 60 K to 80 K, negative thermal quenching phenomena were found for three emission peaks, the neutral donor–bound exciton ( D 0 , X ), the donor–acceptor pair DAP and D complex , in PL spectra of the tail crystal. ICP-MS test, IR transmittance, I–V measurement and IR microscope revealed that the CMT:In crystals with many impurities and defects tended to cause abnormal photoluminescence. Specially, impurities mainly resulted in the formation of intermediate states and were responsible for the negative thermal quenching behavior. The abnormal PL behavior can be explained by intermediate states. • Temperature-dependent photoluminescence properties of indium-doped CdMnTe crystals were investigated. • Negative thermal quenching behavior existed in the range of 20~40 K and 60~80 K for tip and tail crystals, respectively. • Impurities and lattice defects related to intermediate states were responsible for the negative thermal quenching behavior.

Size-dependent luminescence of boron difluoride 1-(2′-naphthyl)butanedionate-1,3

Mechanofluorochromism and size-dependent luminescence properties of boron difluoride 1-(2′-naphthyl)butanedionate-1,3 (1) have been investigated. It has been shown that the evolution of the excitation and luminescence spectra of crystals 1 occurred in accordance with changes in their sizes, regardless of the method of size reduction (rapid crystallization or grinding), which indicates the size-dependent luminescence of 1. When the size of a crystal decreased, a hypsochromic shift of the maxima of the excitation and luminescence spectra was observed, which was associated with a decrease in the size of J-aggregates. According to luminescence microscopic images, the luminescence of crystals at the illuminated position (luminescence of the crystal side face) and at the edge of crystals is differ. The anisotropy of the luminescence properties of the crystals of 1 were investigated by steady-state and time-resolved fluorescence spectroscopy methods.

Fluorine-substituted cyclobutenes in the solid state: Crystal structures, vibrational spectra and mechanical and thermodynamic properties

The crystal structures, infrared and Raman spectra, and mechanical and thermodynamic properties of four important fluorine-substituted cyclobutene derivatives in the solid state are investigated using first-principles solid-state methods based on density functional theory. These compounds are hexafluorocyclobutene (HFCB, C4F6), 1,3,3,4,4-pentafluoro-2-methoxycyclobut-1-ene (PFMCB, C4F5OCH3), 3,3,4,4-tetrafluoro-1,2-dimethoxycyclobut-1-ene [TFDMCB, C4F4(OCH3)2] and 1,2-dichloro-3,3,4,4-tetrafluorocyclobut-1-ene (DCTFCB, C4Cl2F4). Although some of the properties of the corresponding molecules in the gas phase have been studied, and the structures of the corresponding molecular crystals have been determined by refinement from X-ray diffraction data, their vibrational spectra and properties have not yet been reported. The computed crystal structures and X-ray diffraction patterns are in excellent agreement with their experimental counterparts. The infrared and Raman spectra are calculated from the computed crystal structures using density functional perturbation theory. The results are highly consistent with the corresponding spectra measured experimentally in the gas or liquid phases and, therefore, appropriate normal coordinate analyses of the theoretical results are employed to rigorously assign all the vibrational bands. The elasticity matrices of these materials are computed using the finite deformation technique and a complete set of relevant mechanical properties is determined. Their equations of state are also obtained. These compounds are shown to be weak, highly anisotropic materials displaying the negative Poisson's ratio phenomenon. The TFDMCB also exhibits the negative linear compressibility (NLC) phenomenon for external isotropic pressures in the range of 0.64–1.76 GPa. The computed minimum compressibility, found at P = 0.73 GPa, is substantial (k=−192.9 TPa−1). The NLC effect in TFDMCB is due to a collective rotation of the molecules within the crystal under increasing pressure. Finally, the thermodynamic properties of these materials are determined as a function of temperature using phonon calculations. The computed specific heats of HFCB, PFMCB, TFDMCB and DCMCB at T = 250 K are 127.5, 150.4, 169.9 and 134.8 J·K−1·mol−1, respectively, and corresponding entropies are 152.8, 173.3, 195.4 and 186.6 J·K−1·mol−1.

Random photoluminescence emission base-on droplet evaporative segregation of All-inorganic perovskite for All-photonic cryptographic primitive

Segregation is a phenomenon of inhomogeneous distribution of constituent elements during crystallization and could be found in alloy, polymers, colloid, etc. Some researchers have also found component segregation in perovskite crystals. Until now, the existing research reports are all about the halogen segregation of perovskite at the micro level, while the research about the component segregation of perovskite at the macro level is blank, leaving no relevant application developed. Therefore, we can broaden the PUFs-based application of perovskite crystals by employing the multi-wavelength emission property arising from component macrosegregation. We synthesized perovskite crystal arrays with macroscopic composition segregation in one step by surface-tension-confined evaporative self-assembly. The macrosegregation in perovskite crystals prepared with different halogen ratios and temperature was surveyed. The photoluminescence spectra of perovskite crystals were investigated. Perovskite crystals with composition segregation could generate physically unclonable cryptographic primitives, which were made by a simple and random process and difficult to replicate. According to the number of peaks in the spectrum, we converted the optical response into quaternary cryptographic keys, and demonstrated the randomness and stability of the system. This study excavated the application prospect of macroscopic component segregation perovskite in all-photonic cryptographic primitives.

Extraction of carrier concentration and mobility of ZnO by mid-infrared reflectance spectroscopy

As a representative of the third-generation semiconductors, ZnO has great development potential in the fields of optics and electricity, so the measurement on its electrical properties is quite important. Here, we try to extract the carrier concentration and mobility of ZnO by mid-infrared reflectance spectroscopy. Considering that the longitudinal optical phonon-plasmon coupling (LOPC) mode formed by the coupling of ZnO optical phonons and free carriers depends on carrier concentration and mobility, we conduct a test on the MIR spectra (400-1600 cm−1) of ZnO crystals with different carrier concentration (1018-1015 cm−3). By fitting the strong reflection peak shape corresponding to phonon-polaritons formed by the coupling of its LOPC and infrared light through m-CDF model, the carrier concentration and mobility of these samples are obtained, which are consistent with those got from the Hall-effect measurement.

Hydrogen in nonstoichiometric cubic titanium monoxides: X-ray and neutron diffraction, neutron vibrational spectroscopy and NMR studies

Hydrogen-induced changes in the properties of transition-metal oxides have attracted much recent attention due to numerous applications of these materials including catalysis, H2 production, low-temperature H2 sensing, solar cells, and air purification. However, basic properties of hydrogenated titanium monoxides have not been investigated so far. In the present work, we report the results of the first studies of the crystal structure, vibrational spectra, and mobility of H atoms in TiO0.72H0.30 and TiO0.96H0.14 using X-ray diffraction (XRD), neutron powder diffraction, neutron vibrational spectroscopy, and nuclear magnetic resonance (NMR). The hydrogenated compound TiO0.72H0.30 is found to retain the disordered cubic B1-type structure of the initial titanium monoxide, where H atoms exclusively occupy vacancies in the oxygen sublattice. It has been revealed that hydrogenation of the disordered cubic TiO0.96 leads to the formation of the two-phase compound TiO0.96H0.14, where the disordered B1-type phase coexists with the monoclinic phase of Ti5O5 type with an ordered arrangement of vacancies. In both phases, H atoms are found to occupy only vacancies in the oxygen sublattice. The low-temperature inelastic neutron scattering spectra of TiO0.72H0.30 and TiO0.96H0.14 in the energy transfer range of 40–180 meV exhibit a single peak due to optical oxygen vibrations (centered on about 60 meV) and a broad structure at 90–170 meV due to optical H vibrations. The unusual width of this structure can be attributed to the broken symmetry of hydrogen sites in the titanium monoxides: because of the presence of vacancies in the titanium sublattice, the actual point symmetry of these sites appears to be lower than octahedral. Proton NMR measurements have revealed that both hydrogenated compounds are metallic; no signs of hydrogen diffusive motion in TiO0.72H0.30 and TiO0.96H0.14 at the frequency scale of about 105 s-1 have been found up to 370 K.

Quantum cutting properties in KYF4:Tb3+, Yb3+ phosphors: Judd-Ofelt analysis, rate equation models and dynamic processes

Spectroscopic properties play a significant role during the design and development of novel luminescent materials. In this work, quantum cutting (QC) luminescent materials KYF4:Tb3+,Yb3+ phosphors were successfully synthesized by sol–gel method. The crystal structure, morphology, fluorescence spectra, and decay curves were investigated by X-ray diffractometer (XRD), scanning electron microscope (SEM) and spectrofluorometer. The Judd-Ofelt (J-O) parameters, Ωλ (λ = 2, 4, 6), were directly derived from the emission spectra and decay curves, attributed to the transition intensities of Tb3+ ions were dependent on the J-O parameters. Then, the radiative and nonradiative transition rates, fluorescent branching ratios, cross-relaxation rates and lifetimes of QC luminescence were calculated based on the obtained J-O parameters. Moreover, the rate equation models of QC luminescence were established and the time-dependent population densities in excited levels of Tb3+ ions and Yb3+ ions were simulated. Finally, the dynamic processes of QC luminescence were discussed carefully and the corresponding theoretical quantum efficiency was estimated. Our studies would provide an insight into the QC luminescence and give theoretical guidance for developing novel luminescent materials.

Accurate and Efficient Computation of Optical Absorption Spectra of Molecular Crystals: The Case of the Polymorphs of ROY.

When calculating the optical absorption spectra of molecular crystals from first principles, the influence of the crystalline environment on the excitations is of significant importance. For such systems, however, methods to describe the excitations accurately can be computationally prohibitive due to the relatively large system sizes involved. In this work, we demonstrate a method that allows optical absorption spectra to be computed both efficiently and at high accuracy. Our approach is based on the spectral warping method successfully applied to molecules in solvent. It involves calculating the absorption spectrum of a supercell of the full molecular crystal using semi-local time-dependent density functional theory (TDDFT), before warping the spectrum using a transformation derived from smaller-scale semi-local and hybrid TDDFT calculations on isolated dimers. We demonstrate the power of this method on three polymorphs of the well-known color polymorphic compound ROY and find that it outperforms both small-scale hybrid TDDFT dimer calculations and large-scale semi-local TDDFT supercell calculations, when compared to the experiment.

Quaternary Tl2CdGeSe4 selenide: Electronic structure and optical properties of a novel semiconductor for potential application in optoelectronics

We report on a coupled experimental and theoretical study of electronic and optical properties of quaternary Tl2CdGeSe4 selenide. The experimental results reveal p-type electrical conductivity, the band gap energy of Eg ​= ​1.52 ​eV for this compound and its non-direct nature. The latter experimental findings are confirmed theoretically by first-principles calculations based on density functional theory (DFT). The DFT calculations present that Se 4p states give principal contributions to the top of the valence band of Tl2CdGeSe4, whereas the central portion of the band is composed from Se 4p, Ge 4p and Cd 5s states, its lower portion is due to the input of Tl 6s states and the band bottom is dominated by Ge 4s states with smaller Se 4p states contributions. To verify the DFT calculations the XPS spectrum of valence electrons was measured for Tl2CdGeSe4 crystal and XPS core-level spectra were studied to evaluate charge states of the composing atoms and features of the chemical bonding. The present theoretical data of calculations of the principal optical constants reveal that Tl2CdGeSe4 selenide is a very promising semiconductor for optoelectronics. The Tl2CdGeSe4 crystal was found to be rather stable with respect to treatment its surface with middle-energy Ar ​+ ​ions. The crystal surface of Tl2CdGeSe4 demonstrates comparatively low hygroscopicity.

A non-centrosymmetric cocrystal assembled by O[sbnd]H…N and C[sbnd]H…π supramolecular synthons

A novel NLO material, Acridine:Naphthoresorcinol cocrystal (ACRNR) was obtained by solvent assisted grinding method and single crystals were grown from methanol. The charge transfer activity of ACRNR cocrystal was confirmed by UV-Visible study. The photoluminescence (PL) spectrum of ACRNR crystal exhibit violent and green emission at 365 and 551 nm, respectively. The presence of various function groups and intermolecular interaction between acridine and naphthoresorcinol was determined by FT-IR spectral analysis. Single crystal X-ray diffraction analysis shows that ACRNR cocrystal belongs to orthorhombic system with non-centrosymmetric space group, P212121. Further, crystal-packing analysis suggests that there are OH…N and CH…O hydrogen bonds between acid and base moieties in ACRNR crystal. TG/DTA analysis indicates that the crystal is thermally stable up to 173.5 °C. Dielectric and microhardness measurements were performed to investigate the electrical and mechanical properties. Second harmonic generation (SHG) efficiency was identified by Kurtz and Perry technique and compared with KDP. The laser damage threshold value of grown crystal was found to be 3.24 GW/cm2.

Spectra and nature of the electronic states of [1]Benzothieno[3,2-b][1]benzothiophene (BTBT): Single crystal and the aggregates.

Absorption, fluorescence, and phosphorescence spectra of single crystals of [1]benzothieno[3,2-b][1]benzothiophene (BTBT) and BTBT dispersed in frozen n-nonane, n-hexadecane, and dichloromethane matrices were studied at 5K. Observation of a new absorption band and related changes in the fluorescence to phosphorescence intensity ratio, when the concentration of BTBT in the matrix increased above 10-4M, indicated the presence of BTBT aggregates. Quantum-chemistry calculations performed for the simplest aggregate, isolated dimer, showed that its structure is similar to the "herringbone" element in the BTBT crystal unit cell and the lowest electronic excited singlet state of the dimer has the intermolecular charge-transfer character. A qualitatively different nature of this state in dimers and in crystals, when compared with the situation in BTBT monomer [locally excited (LE) state], is associated with a decrease in the intersystem crossing yield. The structured vibronic structure of phosphorescence spectra in the studied systems indicated LE character of the triplet states.

A Quantitative Interpretation for the Difference of Terahertz Spectra of dl- and l-Alanine: Origins of Infrared Intensities in Terahertz Spectroscopy

The absorption spectra of l-, d-, and dl-alanine in the low-frequency region were measured by terahertz (THz) time-domain spectroscopy ( Appl. Phys. Lett. 2005, 86, 053903, DOI: 10.1063/1.1857080). It has been observed that several absorption bands have a significant difference between the enantiomers (l- and d-alanine) and the racemic compound (dl-alanine) in their peak frequencies. In this work, we calculate the THz spectra of dl- and l-alanine by solid-state density functional theory to quantitatively interpret the spectral difference. The two systems give rise to similar distributions of normal modes in the 5–80 cm–1 frequency range; however, the THz spectra of both crystals are different. During our work, we found two critical aspects regarding the intensities of the THz bands. One aspect, as is known for a molecular crystal, is that the cancellation of the transition dipole moments in the unit cell: Even if an individual molecule in a unit cell has a nonzero transition dipole moment, the total transition dipole moment of the unit cell is zero due to the cancellation of the contributions from the molecules. The second aspect is that these THz normal modes of dl- and l-alanine are dominated by the intermolecular translations, which are intrinsically infrared (IR)-inactive if the molecules are not polarized. Our analysis shows that the IR intensities of these “almost-pure” intermolecular translations are due to a subtle balance between the IR activities induced by the polarization effect of translations and the IR activities of the librations and intramolecular vibrations, which have marginal presences in these normal modes.

Polar modes and overtone states in Raman spectra of lithium tantalate crystals

The conditions for the excitation of transverse and longitudinal polar modes in the processes of Raman scattering of light in mono- and polydomain lithium tantalate single crystals are established using a backscattering geometry. The relative intensity of the overtone Raman spectrum on the longitudinal and transverse fundamental polar excitations is much higher in the polydomain sample. Unfortunately, the exact reason for the difference in the Raman spectra of the mono- and polydomain lithium tantalate samples is unknown. The frequencies of the two overtone bands exceeded the exact value of the overtone frequency corresponding to the fundamental longitudinal polar mode $4{A}_{1}(\text{LO})$, which clearly indicates the existence of bound phonon pairs (biphonons). This is consistent with theory and has been shown for both mono- and polydomain samples. On the basis of the general theory of bound states of quasiparticles, the possibility of the existence of bound states of quadrupole excitations of the antipolar type in the vibrational spectrum of a polydomain crystal of lithium tantalate is predicted.

High-pressures study by Raman spectroscopy and DFT calculations of L-tyrosine hydrobromide crystal

The high-pressure Raman spectra of L-tyrosine hydrobromide crystal (LTHBr) were obtained from 1.0 atm to 8.1 GPa in the 100–3200 cm−1 spectral region. The structural conformation and dimensions of the monoclinic unit cell were estimated using the powder X-ray diffraction (PXRD) method and Rietveld refinement using the GSAS program. At atmospheric pressure, the Raman spectrum was obtained in the spectral range of 100–3200 cm−1 and the assignment of the normal modes based on density functional theory calculations was provided. Large wavenumber shifts of modes at 106, 123, and 157 were observed, which were interpreted as the large displacement of the atoms, making the molecule a flexible structure. The change in the slope (dɷ / dP) of these bands between the pressures of 3.0 and 4.0 GPa and the appearance of a mode of low wavenumber indicate the occurrence of a structural phase transition. A band initially observed at 181 cm−1 in the spectrum recorded at 0.7 GPa change the relative intensity with a band at 280 cm−1 (recorded at 5.8 GPa), indicating a conformational transition. In the region of the internal modes, the spectra show changes that reinforce the conformational phase transition since the bands initially at 1247 and 1264 cm−1 observed at 1.0 GPa have their intensities reversed, and at 3.0 GPa it is observed the fusion of the bands at 1264 and 1290 cm−1 (values recorded at ambient pressure). Thus, we can assume that the LTHBr crystal has undergone a structural phase transition and a conformational phase transition in the pressure range investigated.

Remarkable Properties of Isotope Effect

Present paper is devoted to the non - accelerator manifestation of the strong nuclear interaction - the heart of quantum chromodynamics (QCD) which is part of the Standard Model (SM). The observation of isotopic shift (0.103 eV) of the zero - phonon emission line in photoluminescence spectra of LiD crystals (possessing a strict interaction in the deuterium nucleus) comparison with LiH (in the hydrogen nucleus of which there is no strong interaction) is a first direct proof of the strong nuclear long - range character. The non - accelerating measurement of the strong interaction constant from the distance between nucleons made it possible to find the maximum possible value of αs = 2.4680. The isotopic acquisition of mass by massless fermions is briefly discussed

The study of the optical phonon frequency of 3C-SiC by molecular dynamics simulations with deep neural network potential

In this work, we implement molecular dynamics (MD) simulations with deep neural network (DNN) potential trained with the datasets from ab initio calculations to determine the dielectric spectra of crystal. The fluctuations of the total dipole moment of crystal, which are obtained from MD, can be directly related to the frequency-dependent permittivity according to the work of Neumann and Steinhauser [Chem. Phys. Lett. 102, 508–513 (1983)]. We generalize their theoretical work to express the permittivity in the form of a tensor and perform MD simulations for cubic silicon carbide (3C-SiC) with 8000 atoms to assess the accuracy. The infrared resonance frequency and the phonon linewidth obtained by the DNN potential are compared with those obtained by the empirical Vashishta potential and experiments. The results of the DNN potential are in good agreement with the experimental measurements. It shows that we can carry out MD simulations for large systems with the accuracy of ab initio calculations to obtain dielectric properties.